1r.compress(1) Grass User's Manual r.compress(1)
2
6 r.compress - Compresses and decompresses raster maps.
7
9 raster
10
12 r.compress
13 r.compress help
14 r.compress [-uq] map=string[,string,...] [--verbose] [--quiet]
15 Flags:
17 -u
18 Uncompress the map
19 -q
21 Run quietly
22 --verbose
24 Verbose module output
25 --quiet
27 Quiet module output
28 Parameters:
30 map=string[,string,...]
31 Name of existing raster map(s)
32
34 The GRASS program r.compress can be used to compress and decompress
35 raster map layers.
36 During compression, this program reformats raster maps using a run-
38 length-encoding (RLE) algorithm. Raster map layers which contain very
39 little information (such as boundary, geology, soils and land use maps)
40 can be greatly reduced in size. Some raster map layers are shrunk to
41 roughly 1% of their original sizes. Raster map layers containing com‐
42 plex images such as elevation and photo or satellite images may
43 increase slightly in size. All new raster maps are now automatically
44 stored in compressed form (see FORMATS below). GRASS programs can read
45 both compressed and regular (uncompressed) file formats. This allows
46 the use of whichever raster data format consumes less space.
47 As an example, the Spearfish data base raster map layer owner was orig‐
49 inally a size of 26600 bytes. After it was compressed, the raster map
50 became only 1249 bytes (25351 bytes smaller).
51 Raster files may be decompressed to return them to their original for‐
53 mat, using the -u flag of r.compress. If r.compress is asked to com‐
54 press a raster map which is already compressed (or to decompress an
55 already decompressed raster map), it simply informs the user the map is
56 already (de)compressed and exits.
57
59 r.compress can be run either non-interactively or interactively. In
60 non-interactive use, the user must specify the name(s) of the raster
61 map layer(s) to be compressed (or decompressed) on the command line,
62 using the form map=name[,name,...] (where each name is the name of a
63 raster map layer to be compressed or decompressed). The default behav‐
64 ior is to compress the named map(s).
65 FORMATS
67 Conceptually, a raster data file consists of rows of cells, with each
68 row containing the same number of cells. A cell consists of one or
69 more bytes. The number of bytes per cell depends on the category val‐
70 ues stored in the cell. Category values in the range 0-255 require 1
71 byte per cell, while category values in the range 256-65535 require 2
72 bytes, and category values in the range above 65535 require 3 (or more)
73 bytes per cell.
74 The decompressed raster map format matches the conceptual format. For
76 example, a raster map with 1 byte cells that is 100 rows with 200 cells
77 per row, consists of 20,000 bytes. Running the UNIX command ls -l on
78 this file will show a size of 20,000. If the cells were 2 byte cells,
79 the file would require 40,000 bytes. The map layer category values
80 start with the upper left corner cell followed by the other cells along
81 the northern boundary. The byte following the last byte of that first
82 row is the first cell of the second row of category values (moving from
83 left to right). There are no end-of-row markers or other syncing codes
84 in the raster map. A cell header file (cellhd) is used to define how
85 this string of bytes is broken up into rows of category values.
86 The compressed format is not so simple, but is quite elegant in its
88 design. It not only requires less disk space to store the raster data,
89 but often can result in faster execution of graphic and analysis pro‐
90 grams since there is less disk I/O. There are two compressed formats:
91 the pre-version 3.0 format (which GRASS programs can read but no longer
92 produce), and the version 3.0 format (which is automatically used when
93 new raster map layers are created).
94 PRE-3.0 FORMAT:
96 First 3 bytes (chars) - These are a special code that identifies the
97 raster data as compressed.
98 Address array (long) - array (size of the number of rows + 1) of
100 addresses pointing to the internal start of each row. Because each row
101 may be a different size, this array is necessary to provide a mapping
102 of the data.
103 Row by row, beginning at the northern edge of the data, a series of
105 byte groups describes the data. The number of bytes in each group is
106 the number of bytes per cell plus one. The first byte of each group
107 gives a count (up to 255) of the number of cells that contain the cate‐
108 gory values given by the remaining bytes of the group.
109 POST-3.0 FORMAT:
111 The 3 byte code is not used. Instead, a field in the cell header is
112 used to indicate compressed format.
113 The address array is the same.
115 The RLE format is the same as the pre-3.0 RLE, except that each row of
117 data is preceded by a single byte containing the number of bytes per
118 cell for the row, and if run-length-encoding the row would not require
119 less space than non-run-length-encoding, then the row is not encoded.
120 These improvements give better compression than the pre-3.0 format in
122 99% of the raster data layers. The kinds of raster data layers which
123 get bigger are those in which each row would be larger if compressed
124 (e.g., imagery band files). But even in this case the raster data
125 layer would only be larger by the size of the address array and the
126 single byte preceding each row.
127
129 r.support
130
132 James Westervelt,
133 Michael Shapiro,
134 U.S. Army Construction Engineering Research Laboratory
136 Last changed: $Date: 2008-02-01 04:38:08 +0100 (Fri, 01 Feb 2008) $
138 Full index
140 © 2003-2008 GRASS Development Team
142GRASS 6.3.0 r.compress(1)